Typical prior art examples of an exposure apparatus used in the manufacture of semiconductor devices, and the like, are a step-and-repeat exposure apparatus (also referred to as a stepper) in which, while a substrate (wafer or glass plate) is moved step by step, a plurality of exposure areas on the substrate are successively exposed to a pattern on a master plate (a reticle or mask) via a projection optical system, and a step-and-scan exposure apparatus (also referred to as a scanner) in which exposure transfer of a pattern to a plurality of areas on a substrate is repeated by repeating stepping motion and scanning exposure. In particular, the step-and-scan exposure apparatus makes possible the exposure of fine patterns with higher precision and over a greater angle of view because exposure is limited so that use is made only of a portion of the projection optical system that is comparatively close to the optic axis thereof.
These examples of exposure apparatus have a stage device (wafer stage or reticle stage) for moving a wafer or reticle at high speed to achieve positioning. In order to achieve more precise exposure, however, it is necessary to improve the dynamic characteristics of the stage. The reason for this is that occurrence of deviation (error) of the stage relative to a target position manifests itself as a shift in the exposed pattern. A frequency characteristic is a typical example of a dynamic characteristic. The higher the zero-cross frequency, i.e., the control band, of the frequency characteristic, the better the dynamic characteristics obtained.
One example of a technique for improving the dynamic characteristics of a stage is to adapt an arrangement of coarse adjustment (long-stroke) and fine adjustment (short-stroke) stages. Conceptually, this involves an arrangement in which a fine stage having a small output and a short stroke is mounted on a coarse stage having a large output and a long stroke. In general, if a stage under control is made large in size, it is difficult to raise the resonance frequency of the stage structure itself and, as a result, the control band of the stage is limited to low frequencies. A fine stage specialized for microdrive is such that the stage and its actuators can be made small in size, thereby making it easy to raise the control band. If a fine stage is set up on a stage having a large output and long stroke, therefore, it is easy to raise the stage control band. It should be noted that the control band is composed of frequencies in which the open loop gain is 0 dB. For example, when servo-control is considered, the control band means a range of frequencies that the servo system is capable of following up.
In a stage arrangement having the above-described fine/coarse adjustment structure, generally, the positions of the coarse and fine stages are measured by some method, a feedback loop is constructed using these positions and each stage is controlled. Usually, interferometers are used as means for measuring the stage positions. Such a stage arrangement has been proposed in the specification of Japanese Patent Application No. 2002-0007981, which has been filed by the present applicant. Further, measurement of the positions of fine and coarse adjustment members is disclosed in, e.g., the specification of Japanese Patent Application Laid-Open No. 2000-106344.
The fine stage in such a coarse/fine stage device is adapted so as to afford a very short stroke, which is its original purpose. Consequently, if deviation in the degree of parallelism between a guide bar, which guides the movement of the fine stage, and a bar mirror provided on the fine stage in order to measure the position of the fine stage by a laser interferometer exceeds a predetermined value, a change in relative distance between the coarse stage and the fine stage may exceed the limit of the short stroke of the fine stage. In such a case, the fine stage cannot be controlled correctly.
In order to avoid the above-mentioned problem, it is necessary to strictly regulate the direction along which the coarse stage is driven and the surface of the bar mirror so as to render the two parallel, or to assure that the stroke of the fine stage will be large. The former expedient not only increases labor involved in assembly but also is prone to a change in degree of parallelism ascribable to attaching and detaching of the bar mirror, thus making it difficult to achieve good reproducibility. The latter approach represents a departure from the original purpose of the coarse stage and, hence, makes it difficult to enlarge the control band.